soooooooooooooo
would the Massoth Navigator operate exactly the same as my 55015p?
(baby steps)
I switched from the MTS and 55015p system to the Massoth system (massoth navigator, wireless and Massoth 1210Z base unit) this summer. It doesn't work quite the same way and takes a bit of getting used to and quite a bit of setup[ I have 25 locos to enter] , I must admit. I do think it's a better system than the mts one and is still designed for garden operation. You do have to enter all your locomotives onto the system (and you can't have 2 number 3s, or at least with different settings).
When setting up a loco you configure the number, the name, the picture(150ish to chose from preloaded), which functions are active for that locomotive.

You can download the manual from their product page here
8134801 DiMAX Navigator 2.4GHz – Massoth Elektronik GmbH
Programming a loco is a lot easier with the Massoth system as it comes with it's own programming track output, though you will need the Massoth central station or the Piko one to programme with the Navigator.
Programming loco cvs with the 55015 Universal Remote is awkward to do (and iir can't deal with CVs over 99 which is where most of the sound configuration settings are these days)
Wireless range is a lot better - up to 100m, compared to around 10m in my garden with the 55015 on wireless. I think you need the Massoth receiver as well for wireless operation - this should just plug into the LGB bus,
The Piko system is also made by Massoth and is essentially a cut down version, but is a lot cheaper.
i am reluctant to throw money at another toy with which i have fits operating.....no less the Zimo, which i nearly bought a few weeks back, unless i can play with them......
I'd been looking at the Massoth System for years before I eventually pulled the trigger. I agree, it's not easy when you can't try them out. I hope this account helps a little whatever you decide to do.